Method of logging wells



Sept- 1970 u J. wElNTRlT-r 3,525,132

METHOD OF LOGGING WELLS Filed May 14, 1968 2 Sheets-Sheet 2 Eli 210 2,/l2 I3 I4 if l Y I 1 y 5A 5/6 @V6 @M5/NW6 APAC/W Q 45 o 2,5 a 55 4Q wdI""`` s* INVENTOR DOA/M0 J WE//VTF/i BYBAWW'V Q4-K United States PatentO 3,526,132 METHOD OF LOGGING WELLS Donald J. Weintritt, Houston, Tex.,assignor to National Lead Company, New York, N.Y., a corporation of NewJersey Filed May 14, 1968, Ser. No. 729,109 Int. Cl. E21b 49/02 U.S. Cl.73-153 4 Claims ABSTRACT OF THE DISCLOSURE The invention provides animprovement inwell logging, in which the capacity of formation shalesamples to combine with a basic dye such as methylene blue isdetermined, and the capacities are then plotted as a function of welldepth. The results are highly useful in judging the nature of theformations penetrated and are helpful in predicting the nature of theunderlyingn strata.

This invention relates to formation-sampling well logging, and moreparticularly to shale logging, as used in the drilling of oil wells. Y

In drilling wells in search of oil and gas, the costs have increasedsteadily over the years, and at the same' time the ratio of productivewells to dry holesmhas decreased, since the most likely areas forproductive horizons have long since been drilled. It hasaccordinglybecome essential, and indeed indispensible from an economicpay-out standpoint, to derive as much information' as possible from thedrilling of a well, and more particularly, during the actual drilling ofthe well, so that intelligent decisions may be made concerning thedesirability of coring, formation testing, continued drilling, adjustingthe properties, especially the physico-chemical properties, f thedrilling fluid, setting casing, and the like, all of which are expensiveand time-consuming operations, and the proper use of which may bedeterminative for the success of any given well-drilling operation. i

In well-logging operations of various kinds, attentionl may well begiven to shale formations, since in many ways they are almost uniquelydiagnostic for several different kinds of information. For example,their density often has stratigraphic significance, since density mayindicate overburden pressure, aswell as reflecting in some measure theionic content of the water which gave rise to the shale sediment. Shaledensity alone, however, is an insufficient criterion on which to basemany ofthe decisions which must be made concerning the further drillingof the well; and the same is true' for they readiness with which theshale has been drilled, as reflected in the drilling rate. l -f Anobject of the present invention is to provide a novel method of welllogging wherein shale formation samples are tested with an organic dyeand the results plotted quantitatively as a function of depth to give ameaningful result. l v

Other objects of the invention will appear as the description thereofproceeds.

In the drawings, FIGS. 1 and 2 show a number of well logging parametersplotted as a function of depth, for two different wells, exemplifyingthe invention.

' Generally speaking, and in accordance with illustrative embodiments ofmy invention, I collect samples of shale formations obtained from knowndepths, from a bore hole as is drilled in search of oil and gas. Suchsamples are most readily obtained as cuttings brought to the surfaceduring the ordinary course of rotary drilling using circulating drillingfluid. Methods for obtaining such cutting samples and correlating themwith the depth of origin are well known and need not be described here.A typical ICC presentation of the methods used is given in the articleby W. H. Russell entitled Mud and Cuttings Logging which appears aschapter 17 of the book Subsurface Geology in Petroleum Exploration,edited by J. D. Haun and L. W. Le Roy, Golden, Colo. 1958.

Having obtained shale formation samples from known depths in the borehole, I then determine the capacity of the shale sample to combine witha basic dye, such as methylene blue.

While I do not wish to be limited to any theory of operation, it may bementioned that in general, shales comprise a substantial content of clayminerals which have appreciable cation exchange capacity, and whichtherefore combine by cation exchange with relatively large,water-soluble organic cations such as are furnished by the basic dyes.While the cation 'exchange capacity of the clay minerals in the shalesample is very likely a principal factor in the result obtained,nevertheless, the overall texture of the shale, and the medium molecularweight of the basic dyes, as compared with small cations such astetramethyl ammonium on the one hand and very large cations such as thecationic polymers used in water clarication, are important factors whichcontribute to the unique diagnostic value of this particular type ofassay.

The simplest method of determining the capacity of the shale sample tocombine with the basic dye is t0 disperse or slurry the shaleA in water,add enough acid such as sulfuric or hydrochloric so that the pH isslightly on the acid side, and then add a solution of the selected basicdye in small increments to the shale suspension, with stirring aftereach addition. The initial increments of basic dye added will completelybe taken up by the shale, so that while the solution of the basic dyeitself is strongly colored, no color is imparted to the water in whichthe shale is suspended. When in this fashion sutlicient basic dye hasbeen added to the shale sample suspension so that the capacity of theshale to combine with the dye has been exactly matched by the amount ofdyeadded, then additional increments of dye will bring about acoloration of the water of the shale suspension.

The attainment of this equivalence is readily checked by placing a dropof the shale suspension on a piece of white lter paper, and noting ifthe liquid is colored. Generally when this is done, the solids of thedroplet remain in a localized spot on the lter paper, while the waterspreads away from the central spot, so that any coloration in the lattermay be readily detected even though the mass of suspension, whenstirred, .may be dark gray or even black. The drops so removed forthistest are so small in comparison with the usual suspension volume thatthe accuracy of the determination is not affected. Thus, a

sample of the dried shale, add it to from 15 to 25 ml. of

water and agitate. Motor-driven equipment for carrying this out isavailable from laboratory supply houses. Onehalf to 1 ml. of 5 normalsulfuric acid suffices to acidify the shale suspension, and an aqueoussolution of the basic dye, preferably containing 1/100 equivalent perliter, is added in 1/2 ml. increments. The suspension is hand-stirredwith a glass or plastic stirring rod for about one-half minute,whereupon a drop of liquid is removed with the rod and placed on lterpaper. When equivalence has beenreached, the dye will then appear as acolored ring surrounding the solids on the paper.

As mentioned, water-soluble basic dyes generally are useful, such asmethylene blue (922), malachite green (657), auramine O (656), safranineO (841), crystal violet (681), and rhodamine 6G (752). (The numbers inparenthesis following the dye name are the color index number.)Rhodamine 6G gives a striking fluorescence under ultraviolet light,which under some field con- 3 ditions may be advantageous. However,generally speaking, I prefer and fi'nd methylene blue to be best. Itscolor is intense and unmistakable, and it is readily available at knownpurity.

The basic-dye-combining capacity of the shale sample is convenientlyexpressed in milliequivalents of combined dye (dry basis) per 100 gramsof shale sample.

As examples of the usefulness of my invention, reference may ybe made tothe drawings. FIG. l shows welllogging results for a well drilled inNueces County, Tex., for the depth interval of 8,000 feet to 10,500feet. The column designated as 12 shows depth in feet, The first graph10 to the left shows the normalized drilling exponent d as defined inthe article by Jorden and Shirley which appeared in Journal of PetroleumTechnology, November 1966, pp. 1387-1394. (See also U.S. Pat. No.3,368,400.) The next graph 11 is a plot of the drilling rate in feet perhour as a function of depth. The abscissa scales for these graphs, aswell as graphs 14, 16, 17 are given in the headings at the top of thechart. The next graph, 14, shows the percentage hydrocarbon of gas inthe air drawn from the drilling mud exiting from the well, as determinedwith a hot wire gas detector. The readings set forth in the columnmarked 15 are the readings obtained for samples drawn other than duringnormal drilling heading. Thus, TG indicates trip gas while CO indicatesthat returns were being circulated out.

Graph 17 shows the density of the cuttings obtained from the respectivedepths.

Finally, graph 18, directly connected with the present invention, showsthe basic dye combined capacity of the samples. As may be seen from theheading, these are plotted on a scale from -25 equivalents per 100grams, and the actual graph 18 has an excursion between the values ofapproximately 8 and approximately 25 milliequivalents per 100 grams.

The great usefulness of the invention may be seen in this typical plot.For example, at a depth of 9,175 feet, no unusual values were exhibitedby the drilling rate, or by the drilling rate taken together with the dexponent, as appears from graphs and 11 at this depth. However, thebasic dye combining capacity of the shale sample taken at this depth(curve 18) shows an exceedingly high value of 25, much higher than thesamples taken at 9,075 feet, above this depth, and 9,225 feet, belowthis depth, for which samples the basic dye combining capacities wereapproximately l1. At the same time, the shale density at 9,175 feetshowed a minimum, dropping to the low value of 2.36. This in alllikelihood indicates a clay cap layer lof tight, impermeable shale,below which one could expeet to find abnonmal pressure unless it hadleaked olf through faulting or other stratigraphic anomaly. Anothercombination of high basic dye combining capacity and relatively -lowshale density, as compared with samples immediately above and below,occurs at 9,400 feet. Here again, the drilling rate curve isuninformative, even when taken together with the d exponent. The lowexcursions of the basic dye combining capacity curve 18 are also mostinformative. When they occur together with low shale densities, as forexample at 8,475 feet and 8,675 feet, zones of relatively high porosityare indicated.

Further information may be gained for the section from 9,750 to 10,250feet, where the decreasing trend of the shale density combined with theincreasing trend in the basic dye combining capacity indicates thattransition to a zone of abnormally pressured shale is highly probable.In FIG. 2, there is shown a plot of shale density 20 as well as basicdye combining capacity 21 of the shale encountered in the interval 4,000feet to 7,000 feet in a well drilled off the Coast of Louisiana, Anadditional usefulness of the invention may be seen from this figure. Thevery low formation densities at 5,100 and 5,45 O'feet might, withoutother information, be expected to indicate porous sands. The concomitanthigh values of the basic dye combining capacity of about 30 suggest, tothe contrary, that these are water-wet shales which could be expected togive trouble with sloughing when drilled into. This gives the drilleradvance warning that attention should be paid to the mud composition tocounteract this condition, when and if reached.

In the preceding discussions of the curves for shale density and for`basic dye combining capacity, the valuable information which may beobtained by comparing their trends synoptically with depth have beenpointed out. A corollary thereto is that the trend of the differentialbetween the two curves as a function of depth is likewise highlydiagnostic. It is evident that one may feed the two curves into anycommercially available curve plotter and/ or computing device, so as toprovide a plot of the two curves expressed as a differential, eitherarithmetic or geometric, as a function of depth, or in any other waywhich will be evident to those skilled in the art of observing trends inwell logs. Indeed, simply plotting the two curves in adjacent columns,both as a function of depth, as shown in the figures, is a particularlyexcellent method of enabling differential trends t0 be perceived and/ orregistered in any instru-mental fashion desired.

. It may be mentioned that procedures for determining shale density aresimple and well known. The simplest way is to use standard mineralogicaltechniques, by determining the tendency of the sample to sink or tofloat in a series of test liquids of known densities.

While my invention has been described with the aid Ot specific examples,and using specific procedures, it will be apparent that the invention isfundamentally a broad one and that many variations in detail arepossible within the broad scope of the invention as defined in theclaims which follow. i

Having described -the invention I claim:

1. In a process of logging wells wherein selected properties of theformations penetrated are determined and the results plotted in terms ofquantitative values of the said selected properties as a function ofwell depth, the

' improvement which consists in the following:

collecting samples of shale formation from known depths; v

determining the capacities of said samples to react with a basic dye;

determining the densities of said shale samples;

plotting said capacities so determined as a function of well depth;

and plotting said densities of said shale samples on the same graph'with said basic dye reaction capacities.

2. The process in accordance with claim 1 wherein said basic dye ismethylene blue.

3. The process in accordance with claim 1 wherein said plotted densityand said plotted combining capacities are registered so as to display a'differential value derived from said two plottings as a function ofdepth, whereby trends in formation parameters with increasing depth maybe manifested.

4. The process in accordance with claim 3 wherein said Y basic dye ismethylene blue.

2,995,027 8/1961 Bernard et al. 73-153 X JERRY W. MYRACLE, PrimaryExaminer

